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Review

Graphene and Two-Dimensional Materials-Based Flexible Electronics for Wearable Biomedical Sensors

by
Daniel J. Joe
1,
Eunpyo Park
2,
Dong Hyun Kim
1,3,
Il Doh
1,
Hyun-Cheol Song
4,5,* and
Joon Young Kwak
2,6,*
1
Safety Measurement Institute, Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
2
Center for Neuromorphic Engineering, Korea Institute of Science and Technology (KIST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea
3
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
4
Electronic Materials Research Center, Korea Institute of Science and Technology (KIST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea
5
KIST-SKKU Carbon-Neutral Research Center, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea
6
Division of Nanoscience and Technology, Korea University of Science and Technology (UST), Daejeon 34113, Republic of Korea
*
Authors to whom correspondence should be addressed.
Electronics 2023, 12(1), 45; https://doi.org/10.3390/electronics12010045
Submission received: 11 November 2022 / Revised: 10 December 2022 / Accepted: 20 December 2022 / Published: 22 December 2022
(This article belongs to the Special Issue Design, Fabrication and Applications of Flexible/Wearable Electronics)
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Versions Notes

Abstract

:
The use of graphene and two-dimensional materials for industrial, scientific, and medical applications has recently received an enormous amount of attention due to their exceptional physicochemical properties. There have been numerous efforts to incorporate these two-dimensional materials into advanced flexible electronics, especially aimed for wearable biomedical applications. Here, recent advances in two-dimensional materials-based flexible electronic sensors for wearable biomedical applications with regard to both materials and devices are presented.

1. Introduction

Since the discovery of graphene by Novoselov and Geim in 2004 [1], there have been extensive studies and tremendous advances in two-dimensional materials and their families. They have been widely used in industry, scientific research, and medical applications due to their exceptional physicochemical properties, making them desirable candidates for the detection of human vital signs. In particular, the large surface area and planar geometry of 2D materials lead to sufficient surface loading for targeted molecules and excellent conductivity. As tremendous amount of effort is invested in the field of 2D materials, they have shown promising potential in the detection and monitoring of various human physiological signals for high-performance wearable electronics.
Here, we present an overview of two-dimensional materials such as graphene, transition metal dichalcogenides, layered metal oxychalcogenides, and hexagonal boron nitride used to detect human physiological signals, as summarized in Figure 1. In particular, this review will first introduce the physical properties of the 2D materials, which include but are not limited to electrical and optical properties, and also cover piezoelectric and flexoelectric properties. In addition to introducing the 2D material properties, various applications of two-dimensional layered nanomaterials for wearable biomedical applications and their recent advances [2,3,4,5,6,7,8] (classified into two main categories: (1) sensors to measure physical signals including bioelectrical signals, pulse oximetry, and body temperature and (2) sensors to measure chemical signals including glucose and pH sensing) will be discussed.

2. Two-Dimensional Materials with Electrical and Optical Properties

Two-dimensional (2D) materials, including graphene, transition metal dichalcogenides (TMDCs), layered metal oxychalcogenides (LMOCs), hexagonal boron nitride (h-BN), and layered perovskite, have been actively studied over the past few decades. In this section, we will briefly discuss these representative 2D materials and their outstanding characteristics, which allow them to overcome the limitations of conventional semiconductor materials.

2.1. Graphene

Graphene is a single-atom-thick layer of carbon atoms arranged in a 2D honeycomb structure with properties that differ from those of graphite [1]. There have been many past attempts to obtain single-layer graphite, but the interest in 2D materials research has grown exponentially since a high-quality single-layer graphene was obtained by physical exfoliation (Figure 2a) [9]. Graphene film shows outstanding electrical properties, with high mobility (~10,000 cm2/Vs) at room temperature. It also has unique optical properties; for example, it absorbs 2.3% of light in the visible to infrared range, resulting in a highly non-opaque material which can potentially be used in transparent applications (Figure 2b) [10]. Its elastic properties, as demonstrated by Lee et al., include a Young’s modulus of 1 TPa and intrinsic strength of 130 GPa, representing the strongest material ever measured [11]. That the electrical properties of graphene are merely changed when it is stretched suggests a possible use for flexible devices [12,13]. These peculiar properties indicate that graphene can be used in various fields [13,14,15].
One of the proof-of-concept applications of graphene is as a single-molecule detection application. While the bulk properties of traditional materials are not affected by surface absorption, there is no distinction between surface and bulk in graphene [14]. In addition, due to its ambipolarity, adsorption of either electron-withdrawing or -donating materials can affect the conductivity of graphene [9,16]. Based on these properties, Schendin et al. demonstrated the single-molecule sensitivity of NO2 and NH3 for chemical detectors [17].
Other viable applications of graphene include optoelectronics. Wang et al. first demonstrated transparent graphene electrodes with solar cells [18]. Ryu et al. fabricated a graphene touch screen using rapid thermal chemical vapor deposition (RT-CVD) graphene film (Figure 2c). Those researchers employed graphene films in consumer electronics, demonstrating the potential of graphene to replace traditional transparent materials [19].
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Figure 2. (a) Photograph of a large multilayer graphene flake with a thickness of ~3 nm on top of an oxidized Si wafer. Reprinted with permission from ref. [9]. Copyright (2004) American Association for the Advancement of Science. (b) Photograph of an aperture partially covered by graphene and its bilayer. Reprinted with permission from ref. [10]. Copyright (2008) American Association for the Advancement of Science. (c) Photograph of a graphene-based mobile electronic device compared to a traditional transparent material-based device. Reprinted with permission from ref. [19]. Copyright (2014) American Chemical Society.
Figure 2. (a) Photograph of a large multilayer graphene flake with a thickness of ~3 nm on top of an oxidized Si wafer. Reprinted with permission from ref. [9]. Copyright (2004) American Association for the Advancement of Science. (b) Photograph of an aperture partially covered by graphene and its bilayer. Reprinted with permission from ref. [10]. Copyright (2008) American Association for the Advancement of Science. (c) Photograph of a graphene-based mobile electronic device compared to a traditional transparent material-based device. Reprinted with permission from ref. [19]. Copyright (2014) American Chemical Society.
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2.2. MoS2 (a 2D Transition Metal Dichalcogenide Material)

However, due to the absence of a bandgap, the low resistivity of graphene has hindered the development of graphene-based logic transistors [20,21]. Thus, stemming from efforts to engineer graphene with a bandgap, 2D TMDC materials with a bandgap similar to that of silicon have recently been developed. TMDC materials are well known for their applications in solid state lubricants [22,23], photovoltaic devices [24,25], and rechargeable batteries [26,27]. The 2D TMDC materials are generally composed of few-atom-polyhedral layers with transition metal atoms (mainly Mo and W) sandwiched between two layers of chalcogen atoms (typically S, Se, or Te) and MX2 stoichiometry (Figure 3a) [28,29].
MoS2 has become one of the most popular 2D TMDC materials in recent years. It has a bandgap similar to that of silicon (indirect 1.2 eV for bulk and direct 1.8 eV for monolayer) [30,31] and a mobility around ~60 cm2/Vs at room temperature on a SiO2 substrate (Figure 3b,c) [29,31,32,33,34,35]. To produce materials with improved electrical properties, numerous studies on different dielectric materials, such as Al2O3 [36,37], PMMA [38], etc., have been published [39,40]. These studies have demonstrated mobilities of up to ~470 cm2/Vs [38]. Circuit applications such as NOR gates [41], NAND gates [42,43], and SRAM [43] and non-volatile memory devices such as flash memory or two-terminal memristors have been demonstrated using MoS2 [44,45,46,47,48,49,50]. Interestingly, artificial synaptic behaviors with MoS2 have been studied for possible applications in neuromorphic systems [50,51,52,53].
Studies utilizing the bandgap of MoS2 for optoelectronic devices such as photodetectors, LEDs, and solar cells have been performed as well. Yin et al. fabricated a MoS2 phototransistor with high photoresponsivity and fast response and recovery times [54]. While the photoresponsivity of MoS2 is higher than that of graphene, the response and recovery times are lower due to its lower mobility [55,56]. Other applications, including electroluminescence in the form of junction structures such as metal–MoS2 or Si–MoS2 with monolayer MoS2 [57,58] and solar cells, have been demonstrated [59]. Furthermore, device stacks with graphene with high optical absorption show promise for the development of ultrathin and transparent solar cells [60]. Numerous studies with 2D TMDCs (e.g., MoSe2 [61], WSe2 [62], WS2 [63], and MoTe2 [64]) other than MoS2 have also been performed for transistor and optoelectronic device applications [65,66,67,68] to overcome the drawbacks of graphene.
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Figure 3. (a) Schematic illustration of the structure of TMDCs with the formula MX2. (b) Optical image of a monolayer MoS2 particle. (c) Transfer curves measured at room temperature with top gate. Reprinted with permission from ref. [29]. Copyright (2011) Nature Publishing Group.
Figure 3. (a) Schematic illustration of the structure of TMDCs with the formula MX2. (b) Optical image of a monolayer MoS2 particle. (c) Transfer curves measured at room temperature with top gate. Reprinted with permission from ref. [29]. Copyright (2011) Nature Publishing Group.
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2.3. 2D Layered Metal Oxychalcogenide Materials

Other types of 2D materials, such as layered metal oxychalcogenides (LMOCs) [69], group IV monochalcogenides [70], and layered perovskite [71], have recently emerged as 2D materials. Bi2O2Se is an LMOC material used in n-type semiconductors with a high mobility of ~450 cm2/Vs at room temperature; it also exhibits excellent air, moisture, and thermal stability (Figure 4a) [72]. Silicene [73,74] and germanene are group IV materials [75]. A reported silicene transistor shows a mobility around ~100 cm2/Vs at room temperature with an ON/OFF ratio of ~10 (Figure 4b) [73]. Black phosphorus (BP) is a group V layered material; a BP transistor fabricated on SiO2 shows a mobility of around 1000 cm2/Vs at room temperature, which is higher than the mobility of any previously mentioned TMDC material (Figure 4c) [76,77].

2.4. Hexagonal Boron Nitride (2D Insulator)

Lastly, hexagonal boron nitride (h-BN), one of the most studied 2D insulating materials, has a bandgap of around 6 eV [78,79,80]. Since h-BN has an atomically flat surface that is free of impurities and dangling bonds (Figure 5a), it has been used as a substrate and/or dielectric for 2D semiconducting materials. In particular, the small lattice constant mismatch (~1.7%) with graphene opens up many interesting research opportunities [81,82,83,84].
h-BN exhibits a high transparency of 99% in the wavelength range of 250–900 nm (Figure 5b) [85]. Due to its ultraviolet luminescence and direct wide bandgap, h-BN is a promising candidate for deep ultraviolet detectors [86], ultraviolet lasers [87], and single photon emission applications [88,89]. h-BN possesses excellent mechanical properties, with an elastic constant of 220–510 N/m and Young’s modulus of ~1.0 TPa [85,90,91] as well as high endurance and long retention time under bending conditions (Figure 5c) [92,93].
We have briefly gone over some of the electrical and optical properties of 2D materials (graphene, TMDC, LMOC, monochalcogenide, and layered perovskite). Many other characteristics in addition to the aforementioned properties make 2D materials suitable for applications such as FETs, solar cells, photodetectors, and emerging next-generation electronics platforms [94,95,96,97,98,99,100,101,102,103,104,105].
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Figure 5. (a) Schematic of a hexagonal boron nitride (h-BN) showing an optimized atomic structure with two-dimensional h-BN atomic layers. Reprinted with permission from ref. [84]. Copyright (2019) American Physical Society. (b) The UV−visible absorption spectra of the h-BN film showing 99% light transmission in the wavelength range from 250 to 900 nm. Reprinted with permission from ref. [85]. Copyright (2010) American Chemical Society. (c) Image of bending test and results of bending experiment of the h-BN-based resistive memory device. Reprinted with permission from ref. [92]. Copyright (2016) John Wiley & Sons.
Figure 5. (a) Schematic of a hexagonal boron nitride (h-BN) showing an optimized atomic structure with two-dimensional h-BN atomic layers. Reprinted with permission from ref. [84]. Copyright (2019) American Physical Society. (b) The UV−visible absorption spectra of the h-BN film showing 99% light transmission in the wavelength range from 250 to 900 nm. Reprinted with permission from ref. [85]. Copyright (2010) American Chemical Society. (c) Image of bending test and results of bending experiment of the h-BN-based resistive memory device. Reprinted with permission from ref. [92]. Copyright (2016) John Wiley & Sons.
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3. Two-Dimensional Materials with Piezoelectric and Flexoelectric Properties

Piezoelectricity, which can convert mechanical forces or strains into electrical energy and vice versa, is an important material property associated with a limited set of crystals. Piezoelectric materials form the basis of sound and pressure sensors and can generate electricity from small mechanical displacements, vibrations, bending, or stretching to power small electronic devices [106,107,108]. Piezoelectric materials also can directly deform under an external electric field. Due to this simple operating principle, piezoelectric materials occupy the most important position among smart materials that are deformed by an external electric field. Therefore, piezoelectric materials are employed in a wide range of sensor and actuator applications, from MEMS sensors to ultrasonic transducers [109,110].
Generally, piezoelectricity is only possible in materials that have non-centrosymmetric crystal structures with a bandgap. Piezoelectricity comes primarily from the electrical potential induced by the structural asymmetry. The material must also be an insulator to maintain the induced electrical potential. Thus, metals cannot have piezoelectricity, even if they are non-centrosymmetric, since polarization charges are fully screened by high electron concentration.
Recently, piezoelectricity, including flexoelectricity, has been observed in many two-dimensional materials such as hexagonal boron nitride (h-BN), transition metal dichalcogenides (TMDCs), transition metal dioxides (TMDOs), aurivillius structure, etc. [111,112,113,114]. A number of computational predictions for the piezoelectric properties of 2D materials have also been reported through density functional theory (DFT) calculations [115,116,117,118]. Some 2D materials showed a stronger piezoelectric effect than many of the most commonly used piezoelectric crystals. Although the bulk form of the 3D crystals of these materials has no piezoelectric properties at all, they exhibit a strong piezoelectric effect even at the fundamental thickness limit of one or several atoms. Interestingly, a colossal piezoelectric effect can be induced by the nanoscale effect. The discovery of 2D piezoelectric materials paves the way for nanometer-scale miniaturization of familiar piezoelectric technology using low-dimensional materials, which may in turn lead to new types of nanoelectromechanical system (NEMS) devices requiring high electromechanical coupling. In this section, we will briefly discuss the fundamentals of piezoelectricity in 2D materials and review representative 2D piezoelectric materials in terms of the fundamental principle of piezoelectricity.

3.1. Non-Centrosymmetric 2D Piezoelectric Materials

Piezoelectricity originates from the polarization induced by electrical imbalance due to an asymmetric atomic structure. In order for a material to have intrinsically piezoelectric properties, first, the crystal structure must be asymmetric. More accurately, of the 32 crystallographic point groups, 21 are not centrosymmetric, and 20 of them exhibits piezoelectric property (point group 432 is an exception). These 20 point groups with piezoelectricity can be further classified into 10 polar and 10 non-polar groups. The 10 polar groups, called ferroelectrics, have spontaneous polarization, and generally exhibit greater piezoelectric properties. Thus, in order to have piezoelectricity in a 2D material, the crystal structure must be non-centrosymmetric. Due to the structural specificity of a sheet shape, 2D materials can have asymmetric orientation in-plane, out-of-plane, or both in- and out-of-plane. Hence, the polarization direction of 2D piezoelectric materials is determined by the asymmetric atomic orientation.
Figure 6 presents a schematic illustration of the piezoelectricity of 2D crystal structures in terms of polarization direction. Figure 6a,b show monolayer top views and side views of atomically thin hexagonal boron nitride (h-BN) and 2H phase molybdenum disulfide (2H-MoS2) 2D materials with in-plane polarization, respectively [117]. As can be seen from the top view structure, the monolayer h-BN and 2H-MoS2 do not have an inversion center in the in-plane direction. This non-centrosymmetric structure is direct proof that these 2D materials have piezoelectricity. On the other hand, they exhibit perfect symmetry along the vertical z-axis in the side view. Thus, the piezoelectric property is restricted to the in-plane direction (d11) and is not seen in the out-of-plane (d33) direction. The polarization direction is determined by the electric dipole induced by the displacement of cations (B and Mo atoms) and anions (N and S atoms), and the piezoelectric response is strongly dependent on the crystal orientation. Hexagonal h-BN and 2H-MoS2 have the polarization directions of the three-fold symmetry axis in-plane. As such, non-centrosymmetric 2D materials including the hexagonal III-V group and transition metal dichalcogenide (TMDC) monolayers are mostly in-plane piezoelectric.
Figure 6c shows the side view of the out-of-plane 2D piezoelectric material of multilayer alpha-phase indium selenide (α-In2Se3) [119]. The In2Se3 2D layered material has a rhombohedral R3m crystal structure which is non-centrosymmetric. The direction of the electric dipole from a negative selenide atom to a positive indium atom can lead to vertical polarization (d33), as shown by the arrow in Figure 6c. Interestingly, this 2D material exhibits spontaneous out-of-plane polarization, or ferroelectricity, which has been experimentally confirmed by piezoforce microscopy (PFM). Unlike in-plane 2D piezoelectric materials, in this case, piezoelectricity can be preserved in the bulk counterpart if the layered structure is maintained.
Some special 2D materials can exhibit both in-plane and out-of-plane piezoelectricity through synthetic manipulation. Recently, monolayer TMDC 2D materials in which one of the side layers has been replaced with other atoms (Janus 2D materials) have been reported [120]. In particular, monolayer MoS2, which shows in-plane asymmetry, is modified by full replacement of the top-layer S with Se atoms through H2 plasma stripping and thermal selenization, which breaks the out-of-plane inversion symmetry. Hence, the Janus MoSSe 2D material has coupled built-in polarization fields in both the in-plane and out-of-plane directions, which provide an additional degree of freedom for nanoscale sensor and actuator applications.
In order to predict and compare the piezoelectric properties of numerous 2D materials, Blonsky et al. conducted intensive theoretical investigation using DFT calculations [116]. The 2D piezoelectric materials are divided into three different groups, TMDC, metal oxides, and III-V semiconductors (with the III-V semiconductors further subdivided into d11 and d31 groups), in terms of crystallographic structure and polarization direction. The independent piezoelectric coefficients for the elemental combinations of each group were predicted through DFT calculations in terms of stiffness and piezoelectric tensors; the results are summarized in Figure 7. Most of the in-plane 2D piezoelectric materials show higher d11 piezoelectric coefficients than typical bulk piezoelectric materials. In particular, CrTe2 and CdO systems exhibit significantly higher d11 values. On the other hand, the d31 piezoelectric coefficients of III-V semiconductors show very low values compared to those of typical bulk piezoelectric materials. The results of DFT calculations confirm that it is possible to develop and synthesize 2D piezoelectric materials with very high piezoelectric properties for nanoscale applications.
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Figure 6. Top and side views of atomically thin (a) h-BN and (b) 2H-MoS2 [117]. (c) Side views of the atomic structure of layered In2Se3. The B, N, Mo, S, Se, and In atoms are red, blue, silver, yellow, green, and pink, respectively [119]. The arrows show the axes and direction of piezoelectric polarization. Reproduced with permission. Copyright (2012, 2017) American Chemical Society.
Figure 6. Top and side views of atomically thin (a) h-BN and (b) 2H-MoS2 [117]. (c) Side views of the atomic structure of layered In2Se3. The B, N, Mo, S, Se, and In atoms are red, blue, silver, yellow, green, and pink, respectively [119]. The arrows show the axes and direction of piezoelectric polarization. Reproduced with permission. Copyright (2012, 2017) American Chemical Society.
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Figure 7. Periodic tendency for d11 piezoelectric constant in (a) metal dichalcogenides, (b) group II metal oxides, and (c) hexagonal group III-V semiconductors, and d31 piezoelectric constant in (d) hexagonal group III-V semiconductors. Schematic illustration of material structures: (e) 2H, (f) planar hexagonal, and (g) buckled hexagonal structures [115]. Reprinted with permission. Copyright (2015) American Chemical Society.
Figure 7. Periodic tendency for d11 piezoelectric constant in (a) metal dichalcogenides, (b) group II metal oxides, and (c) hexagonal group III-V semiconductors, and d31 piezoelectric constant in (d) hexagonal group III-V semiconductors. Schematic illustration of material structures: (e) 2H, (f) planar hexagonal, and (g) buckled hexagonal structures [115]. Reprinted with permission. Copyright (2015) American Chemical Society.
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3.2. 2D Piezoelectric Materials through Surface Engineering

Generally, a 2D material with inversion symmetry is not intrinsically piezoelectric. However, the symmetry of the crystal structure can be broken by surface engineering, and an intrinsic piezoelectric field can be induced. Piezoelectricity can be created via defects and inclusions even in indisputably non-piezoelectric 2D materials such as graphene. In recent years, Ong and Reed have demonstrated that piezoelectricity can be induced in graphene through chemical doping on one side, which breaks the inversion symmetry [121,122]. They calculated the stress and strain piezoelectric coefficients for graphene with experimentally realizable adatoms {Li, K, H, F, (H, F codoping), (Li, F co-doping)} using DFT. The results show that the highest d31 piezoelectric coefficient in graphene was obtained following F and Li co-doping, whereas the highest e31 piezoelectric coefficient was obtained in the material doped with Li. Likewise, the inversion symmetry of graphene can be broken by creating vacancies, resulting in piezoelectricity. Zelisko et al. found that a graphene nitride (C3N4) nanosheet with naturally created vacancies with triangular geometry experimentally and computationally exhibits an obvious piezoelectric response, as shown in Figure 8a [123]. Even in 2D materials with inversion symmetry, a piezoelectric field can be induced through two techniques, adding atoms or digging vacancies.

3.3. 2D Flexoelectric Materials

The flexoelectricity induced by a strain gradient is worth discussing along with piezoelectricity, considering that the two are functionally the same. Unlike piezoelectricity, flexoelectricity can be induced in all materials with centrosymmetric crystal structures via coupling between the polarization and strain gradients, as shown in Figure 8b [113,124]. Flexoelectricity has scarcely been observed in bulk samples because the magnitude of the effect is seemingly small. However, the large strain gradients often present at the nanoscale can lead to strong flexoelectric effects in low-dimensional nanostructured materials. Recently, flexoelectricity has been discovered in several 2D materials, such as boron nitride, black phosphorus, molybdenum disulfide, and tungsten disulfide [115]. However, the experimental evaluation of these flexoelectric 2D materials that has been conducted is insufficient, and more progress needs to be made. Nevertheless, flexoelectricity is a fascinating form of electromechanical coupling, especially in easily curved 2D materials.

4. Two-Dimensional Materials-Based Wearable Biomedical Sensors

4.1. Wearable Biomedical Sensors for Physical Detection

4.1.1. Bioelectrical Signals

For a portable ECG measurements system, Lou et al. recently demonstrated flexible graphene electrodes made of three electrodes, graphene paper (GP), graphene–polyethylene terephthalate (PET), and graphene textile [125]. As illustrated in Figure 9, graphene–PET- and GP-based electrodes were fabricated, respectively. The authors claimed that experimental results from the ECG recording system demonstrated that all three flexible graphene electrodes could monitor ECG signals with high signal-to-noise ratios (SNRs) in various states of motion. They also asserted that the graphene textile electrodes showed the best flexibility and assembly characteristics when compared to the other electrodes, making them suitable to for a patient’s daily ECG measurements. The flexible graphene electrodes had excellent flexibility, good biocompatibility, comfortability, and high detection sensitivity for reliable bioelectric signal measurements under various states of motion.
Ameri et al. also reported transparent electrodes that resemble tattoos and may be directly laminated onto the surface of human skin, as illustrated in Figure 10 [126]. The sensor shows an optical transparency of over 85%, a total thickness of 463 nm, and a stretchability of more than 40%. Thanks to a conformal contact between the electrodes and human skin, the interfacial impedance of a tattoo made of graphene was comparable to that of conventional silver/silver chloride (Ag/AgCl) gel electrodes. Due to high SNRs, the authors claimed that their graphene-based tattoo-like sensor has been effectively used to monitor electrocardiogram (ECG), electromyogram (EMG), electroencephalogram (EEG), and other electrophysiological measurements.
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Figure 9. (a) Schematic of flexible graphene electrodes; (b,c) photos of graphene–polyethylene terephthalate (PET) and a graphene paper; (d) assembled graphene electrodes; (e) schematic of graphene textile consisting of graphene films and polyester fibers; (f,g) photos of graphene textile electrodes [125]. Reprinted with permission. Copyright (2018) MDPI.
Figure 9. (a) Schematic of flexible graphene electrodes; (b,c) photos of graphene–polyethylene terephthalate (PET) and a graphene paper; (d) assembled graphene electrodes; (e) schematic of graphene textile consisting of graphene films and polyester fibers; (f,g) photos of graphene textile electrodes [125]. Reprinted with permission. Copyright (2018) MDPI.
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Figure 10. (A) Without any skin preparation, the contact impedance between GET and skin is almost identical to that between skin and commercial gel electrodes. (B) EEG sensing using both gel and GET electrodes on the forehead (left). Both spectrograms show an α rhythm of 10 Hz when the eyes were closed. (C) ECG measurements with synchronized GET and gel electrodes. Both electrodes are capable of measuring characteristic ECG peaks. (D) EMG sensing with the GET and gel electrodes on the forearm while the subject squeezed the hand exerciser three times [126]. Reprinted with permission. Copyright (2017) American Chemical Society.
Figure 10. (A) Without any skin preparation, the contact impedance between GET and skin is almost identical to that between skin and commercial gel electrodes. (B) EEG sensing using both gel and GET electrodes on the forehead (left). Both spectrograms show an α rhythm of 10 Hz when the eyes were closed. (C) ECG measurements with synchronized GET and gel electrodes. Both electrodes are capable of measuring characteristic ECG peaks. (D) EMG sensing with the GET and gel electrodes on the forearm while the subject squeezed the hand exerciser three times [126]. Reprinted with permission. Copyright (2017) American Chemical Society.
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4.1.2. Biomechanical Signals

Other types of biomechanical signals of interest can be heartbeat or blood pulses. It is the frequency of cardiac cycles which reflects the heart function of pumping oxygen into blood over the whole human body. Pang et al. reported a reduced graphene oxide (rGO)-based pressure sensor with sensitivity as high as 25.1 kPa−1 in a wide linearity range of 0–2.6 kPa for tiny human biomechanical signal detection [127]. Bioinspired by the microstructure of human epidermis, the interlocked spinosum structure contributed to improvements in changes in contact area and variations of resistance, resulting in the sensor’s high sensitivity, especially in the low-pressure range. According to the authors, their pressure sensor showed superior detecting performance for the heartbeat at both the wrist artery and fingers, thanks to the sensor’s high sensitivity and flexible PDMS substrate.
In addition to measuring blood pulses, breathing activities can also be measured and monitored through 2D-based flexible and wearable sensors. In the case of breath monitoring, Cheng et al. have demonstrated a stretchable and highly sensitive strain sensor based on rGO-composite fibers [128]. According to the authors, the composite fiber exhibited an ultrahigh sensitivity to tensile strain deformation and wide maximal sensing range. The composite fiber sensor also demonstrated quick signal response (time constant less than 100 ms) and excellent reproducibility (cycles up to 10,000 cycles). Due to the microstructure deviations under various mechanical states, the fiber sensor also showed outstanding bending- and torsion-sensitive properties. The graphene-based fiber sensors built into wearables can detect breathing signals for various physical states when attached to the human chest.

4.1.3. Optoelectric Signals

According to Zheng et al., they recently demonstrated a flexible, transparent, highly stable, and ultrabroadband photodetector using large-area and high-crystalline WSe2 films by pulsed-laser deposition (PLD) on polyimide (PI) substrates [129]. According to the authors, the developed WSe2-based photodetector displays superior photoresponsive properties, including an ultrabroadband detection window from 370 to 1064 nm, external quantum efficiency up to 180%, a reversible responsivity of 0.92 A/W, and a fast response time of 0.9 s. It also exhibits an average transparency of 72% in the visible range. The large-area WSe2-based photodetector appears to be particularly promising in wearable optoelectronic applications due to its excellent mechanical flexibility and stability in air as well as its outstanding optoelectronics properties.
Additionally, Song et al. reported a solvothermal method to produce two-dimensional van der Waals heterostructures consisting of transition metal chalcogenides (TMCs) and graphene at a large scale [130]. By merely filtering a large-scale solution processed Bi2Se1.5Te1.5 and graphene nanocomposite (BSTG) structure, a freestanding heterostructured thin film with exceptional mechanical flexibility was fabricated. Flexible optoelectronic applications were made possible by the heterostructure film-based flexible photodetectors, which also exhibited exceptional durability in a bending test.

4.1.4. Body Temperature

Regarding the measurement of one’s body temperature using 2D-based wearable flexible sensors, Yan et al. fabricated stretchable thermistors using resistive graphene as the temperature sensing channels and highly conductive silver nanowires (AgNWs) as electrodes, as shown in Figure 11 [131]. The fabricated devices proved mechanically robust and demonstrated strain-dependent thermal indices, and the sensitivity of the thermistors may be effectively tuned using strain. The authors claim that the tunable thermal sensitivity together with the graphene devices can be adaptable for applications such as temperature sensing in stretchable and wearable electronics.

4.2. Wearable Biomedical Sensors for Chemical Detection

4.2.1. Glucose

A flexible PET substrate-based CVD-grown graphene field-effect transistor (FET)-based glucose sensor was reported by Kwak et al. [132]. The enzymes that cause the catalytic reaction of glucose were immobilized by functionalizing the CVD-grown graphene with linker molecules. The fabricated sensor also showed ambipolar transfer characteristics as well. A Pt wire suspended in a glucose/H2O2 solution was utilized as the top gate electrode to test the responsivity of the sensor. In order to cover the majority of the reference range of medical examination for diabetes diagnoses, a Dirac point shift and linear change in drain/source current were measured to be in the range of 3.3−10.9 mM for glucose detection. Their CVD-grown graphene-based FET sensor developed by the authors may have considerable promise, particularly for portable, wearable, and implantable glucose level monitoring applications.
Additionally, Abellan-Llobregat et al. reported a platinum graphite-based non-enzymatic and enzymatic glucose sensor that is fully printable, cost-effective, and highly stretchable as illustrated in Figure 12 [133]. As an enzyme stabilizer, glucose oxidase (GOx) was immobilized by chitosan and phosphate buffer solution (0.25 M PBS, pH 7.0) after screen-printing procedures on polyurethane (PU) substrate. The range of linear sensitivity for this device was recorded as between 0 mM and 0.9 Mm, and it may be further extended to 75% of its original length. When compared to the non-enzymatic device, this GOx-based enzymatic device yielded excellent results. According to the authors, enzymatic detection significantly enhanced sensitivities and limits of detection (LODs) as compared to the direct oxidation of glucose with Pt–graphite electrodes.

4.2.2. pH Sensing

Meali et al. reported fabricating a pH sensor based on GO to monitor the healing of wounds [134]. The pH-sensitive layer contains a layer of graphene oxide (GO), which was made by casting GO dispersion in drops onto the working electrode of a substrate that had been screen-printed. This pH sensor’s sensitivity was measured to be 31.8 mV/pH with an accuracy of 0.3 pH. According to the authors, the comparison between the results from a typical glass electrode pH-meter and those from their GO pH sensor showed barely detectable differences for measurements taken over a four-day period.

5. Conclusions

Here, we have discussed recent advances and developments in the field of flexible and wearable biomedical sensors using two-dimensional materials. First, an overview of 2D materials including graphene, TMDC, LMOC, and h-BN was presented. Next, their physical properties such as electrical, optical, piezoelectric, and flexoelectric properties were introduced. Finally, several examples of applications and recent advances in wearable biomedical sensors based on 2D materials, such as sensors detecting bioelectrical, biomechanical, and optoelectronic signals, body temperature, glucose, and pH sensing, were discussed.

Author Contributions

Conceptualization, Writing—original draft, and Supervision: D.J.J., H.-C.S. and J.Y.K. Writing—review & editing: E.P., I.D. and D.H.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Development of Measurement Standards and Technology for Bio-materials and Medical Convergence funded by Korea Research Institute of Standards and Science (KRISS–2022–GP2022-0006). J.Y.K. is thankful to the National Research Foundation of Korea (NRF) (grant no. 2021M3F3A2A01037738) and Korea Institute of Science and Technology (KIST) through 2E31550. H.-C.S. would like to acknowledge the support from Korea Institute of Science and Technology (2E31771).

Acknowledgments

This work was supported by Development of Measurement Standards and Technology for Biomaterials and Medical Convergence funded by Korea Research Institute of Standards and Science (KRISS–2022–GP2022-0006). J.Y.K. is thankful to the National Research Foundation of Korea (NRF) (grant no. 2021M3F3A2A01037738) and Korea Institute of Science and Technology (KIST) through 2E31550. H.-C.S. would like to acknowledge the support from Korea Institute of Science and Technology (2E31771).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Summary of applications in the field of biomedical sensors, which can be potentially achieved by flexible electronics-based wearable devices using two-dimensional (2D) materials. Only copyright-free images are included in the figure.
Figure 1. Summary of applications in the field of biomedical sensors, which can be potentially achieved by flexible electronics-based wearable devices using two-dimensional (2D) materials. Only copyright-free images are included in the figure.
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Figure 4. (a) Output curves obtained from a top-gated 6.2 nm-thick Bi2O2Se channel FET and optical microscopy image of a top-gated device. Reprinted with permission from ref. [72]. Copyright (2017) Nature Publishing Group. (b) Buckled honeycomb lattice structure of silicene and R vs. (Vg—Vdirac) of silicene channel FET devices. Reprinted with permission from ref. [73]. Copyright (2015) Nature Publishing Group. (c) Atomic structure of black phosphorus and transfer curves of 5 nm-thick phosphorene FET device. Reprinted with permission from ref. [77]. Copyright (2014) Nature Publishing Group.
Figure 4. (a) Output curves obtained from a top-gated 6.2 nm-thick Bi2O2Se channel FET and optical microscopy image of a top-gated device. Reprinted with permission from ref. [72]. Copyright (2017) Nature Publishing Group. (b) Buckled honeycomb lattice structure of silicene and R vs. (Vg—Vdirac) of silicene channel FET devices. Reprinted with permission from ref. [73]. Copyright (2015) Nature Publishing Group. (c) Atomic structure of black phosphorus and transfer curves of 5 nm-thick phosphorene FET device. Reprinted with permission from ref. [77]. Copyright (2014) Nature Publishing Group.
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Figure 8. (a) A graphene nitride (C3N4) nanosheet, riddled with triangular holes, was experimentally and computationally shown to exhibit an apparent piezoelectric response [123]. (b) The 2D flexoelectric materials with centrosymmetric structures [113,124]. Reprinted with permission. Copyright (2014) Nature Publishing Group, (2015) The Royal Society of Chemistry, and (2013) American Chemical Society.
Figure 8. (a) A graphene nitride (C3N4) nanosheet, riddled with triangular holes, was experimentally and computationally shown to exhibit an apparent piezoelectric response [123]. (b) The 2D flexoelectric materials with centrosymmetric structures [113,124]. Reprinted with permission. Copyright (2014) Nature Publishing Group, (2015) The Royal Society of Chemistry, and (2013) American Chemical Society.
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Figure 11. (a) Images of the stretchable graphene thermistor at 0 (left) and 50% (right) strain are shown. (b) I−V curves of the thermistor at zero strain in the temperature range of 30−100 °C (temperature step chosen as 10 °C for clarity). (c) A non-linear relationship between resistance fluctuation and temperature is shown. (d) A linear relationship between ln(R) and 1000/T is demonstrated [131]. Reprinted with permission. Copyright (2015) American Chemical Society.
Figure 11. (a) Images of the stretchable graphene thermistor at 0 (left) and 50% (right) strain are shown. (b) I−V curves of the thermistor at zero strain in the temperature range of 30−100 °C (temperature step chosen as 10 °C for clarity). (c) A non-linear relationship between resistance fluctuation and temperature is shown. (d) A linear relationship between ln(R) and 1000/T is demonstrated [131]. Reprinted with permission. Copyright (2015) American Chemical Society.
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Figure 12. (A) Diagram showing the design of the stretchable sensor, the immobilization of enzymes on the printed catalytic layer, and the reaction during detection. A real image of a screen-printed sensor is displayed in the inset. (B) Preparing screen-printable, stretchable, and catalytic ink. (C) Screen-printing process illustrating the component layers [133]. Reprinted with permission. Copyright (2017) Elsevier.
Figure 12. (A) Diagram showing the design of the stretchable sensor, the immobilization of enzymes on the printed catalytic layer, and the reaction during detection. A real image of a screen-printed sensor is displayed in the inset. (B) Preparing screen-printable, stretchable, and catalytic ink. (C) Screen-printing process illustrating the component layers [133]. Reprinted with permission. Copyright (2017) Elsevier.
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MDPI and ACS Style

Joe, D.J.; Park, E.; Kim, D.H.; Doh, I.; Song, H.-C.; Kwak, J.Y. Graphene and Two-Dimensional Materials-Based Flexible Electronics for Wearable Biomedical Sensors. Electronics 2023, 12, 45. https://doi.org/10.3390/electronics12010045

AMA Style

Joe DJ, Park E, Kim DH, Doh I, Song H-C, Kwak JY. Graphene and Two-Dimensional Materials-Based Flexible Electronics for Wearable Biomedical Sensors. Electronics. 2023; 12(1):45. https://doi.org/10.3390/electronics12010045

Chicago/Turabian Style

Joe, Daniel J., Eunpyo Park, Dong Hyun Kim, Il Doh, Hyun-Cheol Song, and Joon Young Kwak. 2023. "Graphene and Two-Dimensional Materials-Based Flexible Electronics for Wearable Biomedical Sensors" Electronics 12, no. 1: 45. https://doi.org/10.3390/electronics12010045

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